[MapProxy] WMS source with SRS EPSG:102100, same as 900913

[Josh Doe]

I'm trying to use MapProxy with a WMS that offers the spherical
mercator projection, but it uses SRS code EPSG:102100 instead of
900913. They are equivalent though, as I've detailed here [1].

Is there any way to tell MapProxy that EPSG:102100 needs to be sent to
the server, however it is equivalent to 900913? I'm guessing this will
require a source change.

Thanks,
-Josh

[1]: http://wiki.openstreetmap.org/wiki/EPSG:3857